The Right of Not Turning Left

In 2007 UPS made the headlines by declaring left-hand turns for its army of delivery truck drivers undesirable. Of course, we left-handers have always known that our left or “sinister” side is fatefully less attractive and still branded as unlucky or evil. Chinese culture brands left-handedness as improper as well.

UPS had other motives for poo-pooing left-hand turns. For a company which runs over 95,000 big brown delivery trucks optimizing delivery routes could result in tremendous savings. In fact, careful research showed that the company could reduce its annual delivery routes by 28.5 million miles, save around 3 million gallons of fuel and reduce CO2 emissions by over 30,000 metric tons. And, eliminating or reducing left-hand turns would be safer as well. Of the 2.4 million crashes at intersections in the United States in 2007, most involved left-hand turns, according to the U.S. Federal Highway Administration.

Now urban planners and highway designers in the United States are evaluating the same thing — how to reduce the need for left-hand turns. Drivers in Europe, especially the United Kingdom, will be all too familiar with the roundabout technique for reducing left-handed turns on many A and B roads. Roundabouts have yet to gain significant traction in the United States, so now comes the Diverging Diamond Interchange.

[div class=attrib]From Slate:[end-div]

. . . Left turns are the bane of traffic engineers. Their idea of utopia runs clockwise. (UPS’ routing software famously has drivers turn right whenever possible, to save money and time.) The left-turning vehicle presents not only the aforementioned safety hazard, but a coagulation in the smooth flow of traffic. It’s either a car stopped in an active traffic lane, waiting to turn; or, even worse, it’s cars in a dedicated left-turn lane that, when traffic is heavy enough, requires its own “dedicated signal phase,” lengthening the delay for through traffic as well as cross traffic. And when traffic volumes really increase, as in the junction of two suburban arterials, multiple left-turn lanes are required, costing even more in space and money.

And, increasingly, because of shifting demographics and “lollipop” development patterns, suburban arterials are where the action is: They represent, according to one report, less than 10 percent of the nation’s road mileage, but account for 48 percent of its vehicle-miles traveled.

. . . What can you do when you’ve tinkered all you can with the traffic signals, added as many left-turn lanes as you can, rerouted as much traffic as you can, in areas that have already been built to a sprawling standard? Welcome to the world of the “unconventional intersection,” where left turns are engineered out of existence.

. . . “Grade separation” is the most extreme way to eliminate traffic conflicts. But it’s not only aesthetically unappealing in many environments, it’s expensive. There is, however, a cheaper, less disruptive approach, one that promises its own safety and efficiency gains, that has become recently popular in the United States: the diverging diamond interchange. There’s just one catch: You briefly have to drive the wrong way. But more on that in a bit.

The “DDI” is the brainchild of Gilbert Chlewicki, who first theorized what he called the “criss-cross interchange” as an engineering student at the University of Maryland in 2000.

The DDI is the sort of thing that is easier to visualize than describe (this simulation may help), but here, roughly, is how a DDI built under a highway overpass works: As the eastbound driver approaches the highway interchange (whose lanes run north-south), traffic lanes “criss cross” at a traffic signal. The driver will now find himself on the “left” side of the road, where he can either make an unimpeded left turn onto the highway ramp, or cross over again to the right once he has gone under the highway overpass.